1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spacecraft gyroscopes.
2. Description of the Related Art
Knowledge of spacecraft attitude and the ability to control this attitude are critical features that enable successful operations (e.g., antenna pointing and solar panel orientation) of spacecraft systems. Attitude knowledge and control are respectively realized with attitude detectors and attitude controllers. Spacecraft attitude detectors include sensors (e.g., Sun sensors, Earth sensors and star sensors) which are responsive to celestial radiating bodies and sensors which are responsive to other radiating sources with known locations (e.g., an Earth-based microwave beacon). Spacecraft attitude controllers include torque generators of various types (e.g., momentum wheels, reaction wheels, thrusters and magnetic torquing coils).
Radiating-body sensors provide absolute attitude information. Sun sensors, for example, are typically referred to as single-axis if they detect the Sun's presence along a single inertial plane and as two-axis if they detect the Sun's presence along two intersecting inertial planes. Either type has at least one angular radiation-receiving slot which is usually centered about a selected one of a spacecraft's coordinate axes. The receiving slot must be unobstructed which imposes limitations on other spacecraft structures. In addition, radiating-body sensors occupy spacecraft volume and add to a spacecraft's cost and weight. It is desirable, therefore, to reduce the number and the field-of-view of such detectors that a spacecraft carries.
There are occasions in which absolute attitude information is not available. As a first example, electrostatic thrusters have been proposed for conducting transfer orbits (e.g., from a low-Earth orbit to a geosynchronous orbit). Due to their low thrust levels, the time duration of a transfer orbit may be extensive (e.g., 30-90 days) and preferred flight attitudes may prohibit the use of radiating-body sensors for extended periods (e.g., 2-3 days). As a second example, acquisition time of an Earth-based microwave beacon is sometimes lengthy (e.g., a few hours) and absolute attitude information is absent during this acquisition time. In a final example, failure of one radiating-body sensor may require a change of spacecraft attitude to facilitate the acquisition of another. Absolute attitude information is unavailable for the duration of such an attitude change.
Accordingly, spacecraft are generally also equipped with gyroscopes which provide attitude information without sensing the presence of a radiating body. In contrast, they provide relative attitude information because they sense changes in inertial-space orientation. Spacecraft gyroscopes are fabricated in various types (e.g., rotating-mass gyroscopes, vibrating-mass gyroscopes, ring-laser gyroscopes and fiber-optics gyroscopes) but are typically structured to be responsive to spacecraft rotation about a respective spacecraft axis and to generate an output signal which is representative of this rotation. For example, a rate gyroscope's output signal indicates rotational rate and a rate integrating gyroscope's output signal indicates rotational angle.
Unfortunately, gyroscopes typically exhibit gyroscopic drift, i.e., their output signal indicates rotation when there is none. To enhance the accuracy of the gyroscope's output signal, this gyroscopic drift must be measured and subtracted from the output signal to produce a true rotation signal. Although gyroscopic drift can be determined prior to placement of a spacecraft into orbit, various effects (e.g., radiation, temperature, vibration and aging) alter the drift. As a consequence, gyroscopic drift must generally be established in space and periodically repeated when attitude information is especially critical (e.g., prior to a spacecraft attitude control maneuver.
Methods for calibrating gyroscopic drift in space have often made use of Sun sensors. In one conventional calibration method (see Auburn, J. H. C., et al., "Olympus Manoeuvres in Transfer Orbit-an ESA First", ESA Bulletin, November, 1990, pp. 67-71 and Burton, Michael, AAS 84-001 paper, pp. 3-18), a spacecraft is maneuvered to place a first coordinate axis (e.g., a Z axis of an orthogonal-axes coordinate system) in a Sun-pointing attitude. In this configuration, both the X and Y axes are held orthogonal to a Sun line so that the drift rates of their respective gyroscopes can be calibrated simultaneously.
This configuration is maintained for a time period while the angular estimates derived from the gyroscope measurements are compared to those from the Sun sensor measurements. The difference between the results at the beginning and end of the time period divided by the time difference gives the apparent gyroscopic drift rate. This can be corrected for the apparent motion of the Sun during the calibration period. A similar process is performed for the remaining gyroscope after the spacecraft is maneuvered to place a different coordinate axis in a Sun-pointing attitude so that the Z axis is now held orthogonal to the Sun's direction. Because this calibration process requires a pair of two-axis Sun sensors, it incurs a significant cost, volume and weight penalty.
An exemplary calibration process of U.S. Pat. No. 4,884,771 includes the steps of: a) using a two-axis sensor for measuring the direction of a radiating reference, b) commanding a spacecraft to adopt two different reference attitudes during two consecutive time intervals, c) recording the actual directions of the reference during the two time intervals, d) obtaining the time integrals of the gyroscopes' output signals during the two time intervals and e) determining the gyroscopic drifts from the time integrals representing the gyroscopic drift plus the instantaneous respective spacecraft deviation from the commanded reference orientations and from the sensor measurements. Although this method only requires a single two-axis Sun sensor, its field-of-view must be wide enough to accurately include the two different reference attitudes.